Large plastic articles such as pallets and containers for shipping, require good load bearing properties and are typically flat. Recycled high density polyethylene is plentiful and therefore available at low cost, and beneficially imparts rigidity or stiffness; and is accordingly a desirable thermoplastic material for making this type of article.
Known methods for molding large plastic articles have significant limitations. High molecular weight polyethylene cannot be economically molded by injection molding. Blow molding is not suited for molding a solid thick wall part like a shipping pallet, which often may have a wall thickness of 1/4 to 1/2" or more. Thick wall parts can be rotationally molded from medium molecular weight polyethylene; however, the time to heat and cool a part can run from 30 to 60 minutes.
In addition, available Post Consumer Recycled (PCR) plastics are often mixtures of limited compatibility. Illustrative is a mixture of high density polyethylene (HDPE) and lesser amounts of polyethylene terephthalate (PET) and polypropylene, which are constituents of soft drink containers. Another PCR plastic mixture includes HDPE and linear low density polyethylene (LLDPE). As may be understood, PCR plastic mixtures may range greatly in properties, for instance, density of from 0.915 to 1.20 gm/cc, melt flow of from less than 1 gm/10 min. to more than 30 gm/10 min., and melting point of from 270.degree. F. to 480.degree. F. The percent of shrinkage during cooling of an article made from such diverse mixtures may vary widely, resulting in poor physical properties and appearance due to stress, fatigue and/or delamination. The difficulties are even greater for large articles.
There continues to be a need for low cost, large plastic articles, in particular for large, rigid structures having good load bearing properties. Accordingly, an improved molding process that can take advantage of PCR plastic, especially recycled HDPE, would be beneficial. It would be especially advantageous if a rigid, thick-walled, load-bearing structure could be made at low cost from a PCR resin mixture of limited compatibility.